The present invention is directed to the delivery of data services over a voice communication channel and more particularly to the telephonic delivery of data services by spread spectrum modulation.
The telecommunications industry is currently experiencing a growing demand for systems in which data can be transmitted in analog audio communication channels such as, for example telephone lines during concurrent audio transmissions. These systems substantially improve user connectivity and flexibility as data can be transmitted over existing band-limited links normally used for audio without monopolizing the limited transmission bandwidth available. An important application that can use concurrent audio and data transmission relates to the delivery of off-hook and on-hook data services to telephone subscribers.
As is well known, off-hook data services currently available are all characterized by the exchange of information over an analog audio communication channel (e.g. a telephone line) between a central office and a subscriber already engaged in a telephonic conversation with a third party.
Presently, many telephone companies offer off-hook data services to telephone subscribers. The variety of services is continually increasing and new services are constantly developed to enhance the flexibility and functionality of conventional telephone networks and facilitate the integration of emerging technologies. Examples of off-hook services include call waiting, E-mail messaging, weather and stock reporting services. In addition to off-hook services, many on-hook services are also available such as caller ID and distinctive alerting (or ringing) of different phones attached to the same telephone loop.
With respect to off-hook services, certain constraints affect the service quality that can be offered. Apart from the inherent bandwidth restrictions of conventional subscriber lines, an important constraint affecting off-hook data services relates to the on-going telephonic conversation carried out by the subscriber. Normally, when a subscriber is on a telephone call with a second party, the analog audio signal takes the full bandwidth of the telephone channel. This makes it difficult to transmit data reliably without affecting the on-going conversation between the subscriber and the third party.
A popular technique currently used to transmit data during a telephonic conversation is disclosed in U.S. Pat. No. 4,873,719 entitled xe2x80x9cSpontaneous caller identification with call-waitingxe2x80x9d which issued on Jul. 22, 1991 to Chaput, et al. The method disclosed therein is used to transmit spontaneous call waiting identification (SCWID) information with respect to an incoming call to a subscriber during a phone conversation. According to this method, a short tone is first sent to the subscriber to mute the audio path and the SCWID data is then inserted for transmission during the period of muting. Muting the audio path prevents the SCWID data sent from being distorted by the subscriber""s voice. However, this method causes disruptions in the conversation and can be quite intrusive. It has been shown that with such method, conversations can be disrupted for up to half a second each time information is sent to the subscriber. This can become quite annoying if, depending on the type of service provided, frequent interruptions occur. This would certainly be true of any data reporting service frequently transmitting data to the subscriber. For such services, the method disclosed would not be suitable due to the frequent half-second interruptions that would result.
The present invention addresses these issues and to this end provides a methodology and apparatus to mitigate the present limitations in this art.
The invention provides a method and apparatus for establishing by spread spectrum modulation a non-obtrusive data channel within a voice communication channel to transmit data during an audio transmission. The invention can be incorporated in any communication system where it is desirable to transmit data from a transmitter to a receiver via an analog audio communication channel without disrupting or interrupting any concurrent audio transmission. This audio transmission can be for example, an audio conversation between a first and a second party. By using spread spectrum modulation, the reception and transmission of data can be hidden from the first and second party and allow the ongoing conversation to proceed uninterrupted.
In a preferred embodiment, the invention is used to provide an enhanced call waiting feature whereby caller identifying (ID) information of a third party is transmitted over a telephone line from a local central office (CO) to a subscriber set used by a first subscriber engaged in a telephone conversation with a second subscriber. According to the invention, the local CO and the subscriber set each have in addition to standard telephonic equipment, a pair of analog-to-digital (A/D) and digital-to-analog (D/A) converters, a digital signal processor (DSP) block and a microprocessor.
At the CO, the caller ID information is assembled in the microprocessor into a data message which is then passed on to the DSP block in the form of a digital bit stream. In the DSP block, the digital bit stream is digitally spread (or expanded) by assigning a larger code to each xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d in the stream to produce a spread spectrum modulated signal. This spread spectrum modulated signal is then used to modulate a carrier located within the telephone line pass band. Preferably, a quadrature phase shift keying (QPSK) modulation scheme is used for this. The carrier modulation process is performed by the DSP block which then supplies the resulting data stream to the D/A converter for transmitting the spread spectrum modulated data over the telephone line to the first party. The spread spectrum modulated data is transmitted superimposed on the audio information to form a subscriber line signal.
At the subscriber set, the subscriber line signal is continuously monitored by the DSP block and the microprocessor during an on-going audio call in the event any spread spectrum modulated data is sent by the local CO. For this, the subscriber line signal is digitized by the A/D converter and passed to the DSP block. In the DSP block, the digitized subscriber line signal is initially band pass filtered, QPSK demodulated and subsequently spread spectrum correlated to retrieve the data message sent by the local CO. The data message can then be processed by the microprocessor to extract and display the caller ID information to alert the first party that a third party is calling.
Advantageously, the use of spread spectrum modulation to transmit data in a voice channel carrying a telephone conversation prevents any interruption in the on-going conversation and does not result in any noticeable disruption. In the preferred embodiment, the information relating to the third party can advantageously be presented to the first party engaged in the telephone conversation in a very non-obtrusive and transparent manner. With the exception of a minimal amount of xe2x80x9cwhite noisexe2x80x9d in the background, the first party will not be aware that data is being received or sent.
The invention can advantageously be used for a variety of off-hook services including, but not limited to call waiting, E-mail messaging, weather and stock reporting services. In addition, the invention can also be used for many on-hook services such as caller ID and distinctive alerting (or ringing) of different phones attached to the same telephone loop.
Preferably, for any spread spectrum modulated data transaction between the CO and the telephone set, the CO initially transmits at a nominal power level to accommodate any subscriber loop length. When spread spectrum modulated data is transmitted, the subscriber set operates to measure the power level of the data received. Based on the measurement made, the subscriber set can instruct the CO to raise or lower the spread spectrum modulated data transmit power for optimal reception at the subscriber set.
According to the invention, the CO transmit power can also be controlled without any measurement feedback from the subscriber set. Instead, dual tone multi-frequency (DTMF) signals sent by the subscriber set as it receives a telephone call can be used by the CO to adjust its spread spectrum modulated data transmit power. According to this method, the CO can measure the level of the DTMF signals it receives from the subscriber set using known DSP techniques to estimate the length of the subscriber""s telephone line. With knowledge of the approximate subscriber line length, the CO can then send spread spectrum modulated data at the appropriate transmit power level for optimal reception at the subscriber set.
By controlling the transmit power, any variation of the transmission conditions such as loop length variations can advantageously be compensated such that at the subscriber set, the spread spectrum modulated data is always received at an optimum level.